1. Field of the Invention
The present invention relates to an illuminator that is composed in combination of: a white light source and a sheet condensing lens.
2. Description of the Related Art
In a conventional illuminator that has been generally known, a sheet condensing lens such as a Fresnel lens (hereinafter referred to as the “lens sheet”) is arranged in front of (or on the optical axis of) a light source, so that the orientation of outgoing lights is controlled contributing to a high illumination (or a high brightness). See, for example, Japanese Patent Application Laid-open No. 2002-221605 (hereinafter referred to as the “Patent Document”).
Considering an illuminator disclosed in the Patent Document, as shown in FIG. 11, it is composed as that a lens sheet (a Fresnel lens) 72 is arranged in front of a linear light source 71 (or, at the upper portion of the FIG. 8). The lens sheet 72 has a plurality of refraction prisms (a Fresnel lens) at a center region thereof which is the side of an optical axis 73, the refraction prisms having refraction effects. On the other hand, at or near the outer circumference of the lens sheet 72, a plurality of reflection prisms (referred to as a TIR lens or a Total Internal Reflection lens) having reflection effects is formed.
As discussed hereinabove, since the lens sheet 72 has the refraction prisms at the center region thereof and the reflection prisms at the outer circumference region thereof, compared to the lens sheet where either the refraction prisms or the reflection prisms are individually used, it is possible to obtain luminous lights of high efficiencies due to high illuminations by having large intensified outgoing lights. Also, the intensity of the luminous lights is well homogenized. This is why outgoing lights that have been refracted by the refraction prisms tend to have a large intensity at the center of the lens sheet, but the intensity tends to decrease at the outer circumference of the lens sheet. On the contrary, the outgoing lights that have been reflected by the reflection prisms tend to have a small intensity at the center of the lens sheet, but the intensity tends to increase at the outer circumference of the lens sheet.
In recent years, there are notable demands on illuminators such as a downlight or a spotlight, which use a compact LED (Light Emitting Diode) with excellent environment compatibilities. Considering LEDs which supply white lights, a so-called pseudo-white LED has been widely used. This pseudo-white LED is composed of the following parts in combination: an LED chip (luminous element) that emits a blue-series light (the center wavelength of 410 nm to 480 nm); and a yellow phosphor that absorbs the blue-series light and converts the blue-series light into a yellow-series light (the wavelength range of 480 nm to 700 nm).
Here, the present inventors have constructed an illuminator by combining a pseudo-white LED of a surface mounting type and a lens sheet for a point light source (LED) disclosed by the Patent Document (see the section [0046]). With this illuminator the inventors could obtain luminous lights with a high illumination; however, the inventors observed color shadings in the luminous lights resulting in poor visibility. More specifically, the lights that have been passed through the lens sheet are recognizable as a white light as a whole; however, the lights become somewhat bluish at the center region of the lens sheet while the lights become somewhat yellowish at the outer circumference region of the lens sheet.
The cause of the color shadings will be explained as follows. As shown in FIG. 12, blue lights will be emitted from an LED chip 83 that is mounted on an electrode terminal 82 and that is placed on the bottom surface of the concave portion of a lamp house 81. Among the emitted blue lights, there are a light L1 travelling approximately parallel relative to an optical axis and a light L2 travelling in inclination relative to the optical axis. These lights L1 and light L2 have different optical path lengths, the optical path length being defined by light passing through a sealing body 85 including a plurality of yellow phosphors 84 therewith. To be more specific, the light L1 has a shorter optical path length than the light L2 (meaning that the light L1 has a shorter distance than the light L2 when passing through in the sealing body 85) whereby the light L1 has a relatively small ratio of being converted into a yellow light. The light L1 is thus a bluish white light. On the other hand, the light L2 has a longer optical path length than the light L1 (meaning that the light L2 has a longer distance than the light L1 when passing through in the sealing body 85) whereby the light L2 has a relatively large ratio of being converted into a yellow light. The light L2 is thus a yellowish white light.